1. Field of the Invention
This invention relates to a process for microbial remediation of polluted soil using a microorganism and also a process for treating soil.
2. Related Background Art
With the rapid progress of science and technology, man has been enjoying a living environment in various senses, for example, travelling means, communication means etc., not to mention food, clothing and housing. On the other hand, the natural environment including atmosphere, hydrosphere, and terrestrial zone has been gradually but globally destroyed by scientific and technological results such as exhaust gas of fossil fuel, artificial chemical materials or synthetic compounds and so on. Land has been most affected by those destructive elements because it is the living place for man. Considering that water circulates from land to air and vice versa, it can be said that the terrestrial pollution which will eventually expand to the global level is a serious problem. The now well-known pollutants in soil (land) include, for example, hydrocarbons like gasoline, hydrocarbon halides like PCB, agricultural chemicals having teratogenic activity like dioxin, radioactive compounds and so on. Above all organic compounds are utilized in great quantity as, for example, motor gasoline, detergent solvent for precision mechanical parts, solvent for dry cleaning, and chemicals for insecticides and herbicides. Even now, the pollution of soil with these compounds is in progress. Further it has been found that some organic compound pollutants are strong carcinogens, so that they have grave influence on the biological world. Thus pollution of soil with organic compounds is the problem which should be solved without a moment's delay.
The processes for remedying soil polluted with organic compounds include, for example, a method to heat the excavated soil, a method to conduct vacuum extraction, a method to utilize microorganisms capable of decomposing pollutants, and so on. Heating treatment can remove the pollutant almost completely from the soil. It is, however, not applicable to the purification of the soil under buildings because such soil cannot be dug up. In addition, it is not applicable to the purification of the soil in a wide area, because soil excavation and heating treatment cost comparatively high. Vacuum extraction is low in cost and easy to handle. It, however, requires a very long time to decrease the concentration of pollutants, when the concentration of pollutants in soil is low. Thus it is advantageously used when the concentration of pollutants in soil is high. When utilizing a process which uses microorganism, on the other hand, it is easy to purify the soil under buildings because there is no need to excavate of the soil. Moreover, if microorganisms having a high activity of decomposing pollutants are employed, the purification of polluted soil can be done within a short time even when the concentration of organic compounds is low. Thus this process is now attracting considerable attention as an eco-friendly process.
Conventionally, processes for purifying polluted soil with microorganism have been classified into two types; one is those utilizing native microorganisms which originally inhabit the soil, and the other is those utilizing foreign microorganisms which do not originally inhabit the soil. When the former processes are employed, purification is conducted by injecting the soil with such substances as nutrient, inducer, oxygen and other chemicals to increase the decomposing activity of the microorganism. When the latter are employed, purification includes the step of introducing into the soil foreign microorganism as well as the step of injecting the soil with nutrient and so on which heightens the decomposing activity of the microorganism. When a polluted soil is purified using microorganism, it is desirable that decomposing activity of microorganism be kept high in the wide area of the soil for a long period of time, which is, however, hard to accomplish by the present technique of controlling microorganism. Considering all the factors such as cost, time, safety and so on, the most efficient process for purifying soil is to limit the area to be remedied and to control the decomposing activity of microorganism within the limited area.
One of the prior arts related to the above is U.S. Pat. No. 5,133,625 which describes the method of controlling injection pressure using an extensible injection pipe while measuring injection pressure, flow rate and temperature. This method aims to keep decomposing activity of microorganism optimum by controlling the concentration of the microorganism and nutrients by adjusting the injection pressure. In other words, the object of the prior art is to provide a method for controlling the purification process which utilizes microorganism.
U.S. Pat. No. 4,442,895 and U.S. Pat. No. 5,032,042 disclose the method where the soil is cracked by injecting a gas or a liquid from an injection well by applying pressure, and it says that oxygen and nutrients required for microbial purification can be supplied in this step. The object of this prior art is to provide a process for cracking soil as wide as possible, but not to limit the area to be remedied. U.S. Pat. No. 5,111,883 discloses the method of injecting liquid chemicals into the soil at horizontally and vertically determined sites by setting the relative position of injection well and extraction well. This method aims to provide a process for injecting liquid chemicals into a limited area of the soil in a geometrical manner. It is considered as a very useful method when applied to microbial purification of soil because it can define the area of the soil to be remedied. The problem of the method, however, is that structure of the equipment is very complex.
In order to inject microorganism or substances for maintaining high decomposing activity of the microorganism into a limited area of soil, one of the methods is to form an impermeable layer as a barrier in the soil at a certain distance from the injection well. Conventionally known methods to form such an impermeable layer include laying plastic sheets or forming an asphalt layer in the soil, and injecting the soil with a treating agent such as cement, water-glass, urethane, acrylic amide, acrylate and so on. Japanese Patent Publications No. 2-26662 and No. 5-27676 disclose a method of forming an impermeable layer in a certain soil area using a water soluble polymer which turns water insoluble due to the ions in the soil. This method provides an impermeable layer as a barrier which limits the movement of substances and could be applicable to the process for injecting microorganism and nutrients into the limited area of the soil. Laying plastic sheets and forming asphalt layer in the soil, however, are incompatible with the aim in utilizing microorganism for remedying polluted soil because digging up and returning of soil are required. The process for injecting soil with treating agents such as cement and water soluble polymers requires many injection wells to form a complete barrier, therefore it is not applicable to the microbial remediation of soil for an economical reason. When a foreign microorganism is injected into the soil, the uniform distribution of the microorganism would be very difficult in the soil even using the above technique. Accordingly, an excessive amount of microorganism must be injected so that the polluted soil may be satisfactorily remedied even at the part where the injected microorganism is most difficult to distribute. This leads to an increase in cost of polluted soil remediation.